The present invention relates to a position controlling apparatus for performing the positioning of an object being moved with a high degree of precision.
A typical position controlling apparatus will now be explained with reference to FIG. 1. The position controlling apparatus includes a position controller 1, a velocity controller 2, a motor drive 3, a motor 4 for driving a load 5 that is an object to be controlled, a position detector 6 such as a pulse encoder that is provided to the motor 4, a velocity computing unit 7, and subtracters 8 and 9. The subtracters 8 and 9 may be called a first subtracter and a second subtracter, respectively. The load 5 is, for example, an Xxe2x80x94Y table for moving a piece to be worked on in the X and Y directions. In this case, as the motor 4, a servo motor is suitable.
The function of the position controlling apparatus will hereafter be explained. From a setting unit not illustrated there is given a command value Xr of position. The position detector 6 detects the displacement of the object to be controlled to thereby output a detected value Xm of position. The velocity computing unit 7 differentiates the detected value Xm of position to thereby compute a detected value of velocity. The subtracter 8 subtracts the detected value Xm of position from the command value Xr of position to thereby compute a position error. The position controller 1 amplifies the position error to thereby compute a command value of velocity and outputs this value. The subtracter 9 subtracts the detected value of velocity from the command value of velocity to thereby compute a velocity error. The velocity controller 2 amplifies the velocity error to thereby compute a command value ic of current. The command value ic of current that has been computed is applied to the motor drive 3. The motor drive 3 drives the motor 4 in accordance with the command value ic of current.
FIG. 2 is a block diagram of FIG. 1. The symbol Gp (z) in FIG. 2 represents a transfer function of the position controller 1. Gv (z) represents the transfer function of the velocity controller 2. Kt represents a torque constant of the motor 4. (1/JS2) represents the transfer function of the object to be controlled. (1 -z-1) represents the transfer function of the velocity computing unit 7. Here, it is assumed that xcfx84d represents a disturbance torque. In this case, since the object to be controlled is driven by a sum of the motor torque xcfx84m and the disturbance torque xcfx84d an error occurs when the positioning operation is performed.
In order to decrease the occurrence of the error due to the disturbance torque xcfx84d, it has been considered appropriate to adjust the position controller 1 or velocity controller 2. However, when increasing the gain of the position controller 1 or velocity controller 2 in order to obtain the effect of suppressing the disturbance, the control system is likely to become unstable. This is due to the mechanical vibrations of the object to be controlled or to a phase lag in the sampling period of the position controller 1 and the velocity controller 2.
On the other hand, the detected value Xm of position contains measurement noises Xn. These measurement noises Xn also cause the occurrence of errors at the time of the positioning operation. The measurement noises Xn become a factor that makes the control system unstable when having increased the gain of the position controller 1 or velocity controller 2. Especially, in a digital control system, the noises that are generated when performing the quantization of a sampling frequency band and when performing differentiation in the velocity computing unit 7 are high in level.
Therefore, an object of the present invention is to provide a position controlling apparatus which is capable of the effect of disturbance as small as possible by estimating a disturbance torque acting upon a drive mechanism and compensating for the torque correspondingly thereto in the position controlling apparatus.
Another object of the present invention is to eliminate the measurement noises when performing position detection to thereby prevent degradation of a disturbance suppression characteristic within a high-frequency band.
Still another object of the present invention is to reduce the effects of the measurement noises and differentiation noises within a sampling frequency band especially in a digital control system.
A position controlling apparatus according to the present invention comprises a position detector for detecting the position of a driven object to be driven by a motor, to thereby output a detected value of position. A Kalman filter estimates the position and velocity of the driven object from the detected value of position, thereafter outputting an estimated value of position and an estimated value of velocity. A disturbance estimating unit estimates the disturbance applied to the driven object from comparison with a command value of current and the estimated value of velocity, thereafter outputting this estimated current value of disturbance. A first subtracter computes a difference between a command value of position and the estimated value of position. A position controller computes a command value of velocity in accordance with the difference computed by the first subtracter. A second subtracter computes the difference between the estimated value of velocity and the command value of velocity. A velocity controller computes a target value of current in accordance with the difference computed by the second subtracter. A third subtracter computes a difference between the estimated current value of disturbance and the target value of current to thereby output this difference as the command value of current. The computed command value of current is applied to the disturbance estimating unit and a motor drive for driving a motor.